Different from polymer molded products such as films, fibers, injection molded products and extrusion molded products, polymer microparticles are used for modification and improvement of various materials by utilizing the large specific surface area and the structure of microparticles. Their major uses include modifiers for cosmetics, additives for toners, rheology modifiers for paints and the like, agents for medical diagnosis and examination, and additives for molded products such as automobile materials and construction materials.
On the other hand, with interest growing in recent environmental problems, there are increasing demands for using materials of non-petroleum origin to reduce environmental loads, even in the fields where polymer microparticles are used such as cosmetics and paints. Polylactic acid is one of the representative examples of such non-petroleum origin polymers.
As a conventional method of producing polylactic acid-based resin microparticles or powders, there are several known methods: for example, crushing methods (JP-A-2000-007789 and JP-A-2001-288273) typified by freeze crushing method; dissolution-deposition methods (JP-A-2005-002302 and JP-A-2009-242728) in which deposition is performed by being cooled after being dissolved in a solvent at a high temperature, or in which deposition is performed by adding a poor solvent after being dissolved in a solvent; and melt-kneading methods (JP-A-2004-269865 and JP-A-2005-200663) in which a resin compound containing both a polylactic acid-based resin in dispersed phase and an incompatible resin in continuous phase is formed by kneading the polylactic acid-based resin together with the incompatible resin in a kneading machine such as a two-axis extruder, and in which the incompatible resin is removed subsequently to produce polylactic acid-based resin microparticles.
However, the polylactic acid-based resin microparticles produced by the above-described methods have several problems in that the particles produced are not spherical in shape, particle diameter does not become smaller, particle diameter distribution is broad and, in some cases, it is impossible to keep the particles in a round shape because of fiber-shaped ones or the like. Particularly, in the fields such as cosmetics where great importance is attached to feeling of touch and impression, or in the fields such as paints where it is important to control rheology, effects produced by adding such microparticles were not sufficient hitherto.
On the other hand, as a method for production of polymer microparticles, the method described in WO 2009-142231 is known as a method utilizing emulsion. However, in WO '231, a concrete example of polylactic acid-based resin is not disclosed and it is not clear how to produce polylactic acid-based resin microparticles.
It could therefore be helpful to provide a process of producing polylactic acid-based resin microparticles, porous polylactic acid-based resin microparticles which have small average particle diameter and high oil absorption ability and are appropriately usable for cosmetics and the like, and smooth surface polylactic acid-based resin microparticles which have spherical shape and narrow particle diameter distribution and are appropriately usable for toners and the like.